Metallic film adjustable resistor and contact brush therefor



" June 1957 o. F. KUHLMAN 2,794,892

METALLIC FILM ADJUSTABLE RBSISTQR AND CONTACT BRUS'I THEREFOR Filed NOV. 4, 1954 United States Patent METALLIC FILM ADJUSTABLE RESISTOR AND CONTACT BRUSH THEREFOR Ora F. Kuhlman, Manhattan Beach, Caliii, assignor to Servomechanisms, Inc, a corporation of New York Application November 4, 1954, Serial No. 466,805

11 Claims. (Cl. 201-55) This invention relates to metallic fihn adjustable resistors and contact brushes therefor and, more particularly, to such resistors of the type in which the contact brush bears directly on the resistance film.

Adjustable resistors comprising extremely thin films of stable non-oxidising metals thermally evaporated in a vacuum upon an insulation form, such as a glass plate, have been found useful in applications requiring precisely predetermined resistance-displacement characteristics and stability over a wide range of ambient conditions and over a long life cycle. Because of the extreme thinness of the metal films of such resistors, often of a thickness of only a few molecules, it has heretofore been considered impractical to utilize a contact brush or element bearing directly on the film, since the wear of the film by such contact element, while microscopic, may nevertheless be an appreciable fraction of film thickness, thus substantially changing the resistance characteristics of the unit with use. To avoid such an effect, it has heretofore been proposed that the thin resistance film be superposed upon a commutator structure and that contact be made with portions of the commutator ele ments extending beyond the resistance film. Resistors of such a construction are described and claimed in the patent of David W. Moore, Jr., Reissue No. 23,219, now abandoned, and in the copending application of David W. Moore, Jr., Serial No. 151,430, filed March 23, 1950, now Patent No. 2,720,512.

While the adjustable film resistors described in the afore said patent and patent application are entirely satisfactory for many applications, they have certain limitations. For example, they are relatively costly to fabricate and they have a finite resolution limited by the spacing of the com mutator elements. The present invention represents an improvement on the resistors described in the aforesaid patent and patent application which overcomes their foregoing limitations.

It is an object of the present invention, therefore, to provide a new and improved metallic film adjustable resistor of relatively simple and inexpensive construction.

It is another object of the invention to provide a new and improved metallic film adjustable resistor having substantially infinite resolution.

It is a further object of the invention to provide a new and improved metallic film adjustable resistor having one or more of the following advantages: a long life cycle, minimum contact noise, and maximum current-carrying capacity and power rating.

It is another object of the invention to provide a new and improved contact brush for use in metallic film adjustable resistors of the type described and contributing to aforesaid advantageous characteristics.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, while its scope will be pointed out in the appended claims.

In accordance with the invention, there is provided a metallic film adjustable resistor comprising a form of insulation material having a plane surface, an elongated strip of thin metallic film formed directly on and bonded to said plane surface, and a contact brush formed of a flat slab of graphite having a body portion and tapered edge portion terminating in a substantially line-contact edge, such body portion having a low-resistance metallic coating. The resistor also includes an adjustable holder for moving such brush along the film strip and holding it in a plane substantially normal to the plane surface and to the tangent of the mean path of movement.

Further in accordance with the invention, there is provided in a metallic film adjustable resistor, 21 contact brush comprising a fiat slab of graphite having a body portion and a tapered edge portion terminating in a substantially line-contact edge, the body portion having a low-resistance metallic coating and the edge portion being uncoated and impregnated with a hardening nonconductive sealing substance.

Referring now to the drawing, the single figure is a perspective view of a metallic film adjustable resistor embodying the invention and incorporating a contact brush embodying a feature of the invention.

In the drawing there is illustrated a metallic film adjustable resistor comprising a form of insulation material, such as a circular disc 10 of glass or ceramic having an accurately plane upper surface 10a. On the surface 10a is formed an elongated arcuate resistance strip of thin metallic film 11 having low-resistance terminal coatings 11a, 11b for external connections. The film 11 may be of a noble metal, a nickel-chromium alloy, or other stable substantially non-oxidising metal or metallic alloy. It is preferably formed directly on the surface 10a by thermal evaporation in a vacuum, as well understood in the art, so that it is firmly bonded to the surface 10a. Alternatively, the resistor element 10, 11 may be of the type described and claimed in the copending application of Moore and Kuhlman, Serial No. 466,806, filed concurrently herewith.

The resistor of the invention also includes a contact brush 12 in the form of a flat slab of graphite having a body portion 12:: and a contact edge portion 12b which may be tapered, as shown, terminating in a substantially line-contact edge 120. For supporting the brush 12, there is provided an adjustable radially extending holder 13 extending from a hub 14 pivoted on a shaft 15. The brush holder 13 is thus effective to move the brush 12 along the film strip 11, while holding it in a plane substantially normal to the plane surface 10a and also substantially normal to the tangent 16 of its mean arcuate path of movement 17 which coincides with the center of the film strip 11. In other words, the brush 12 is at all times maintained substantially radial to the film strip 11.

The brush holder 13, 14 includes spring means for biasing the brush 12 vertically into contact with the film strip 11, while restricting brush movement to a vertical plane. This spring means may be in the form of a wire or leaf spring 18 secured to the hub 14 and engaging a hole 12d in the brush element, the spring 13 being under stress to bias the brush 12 vertically downward into engagement with the film strip 11. The spring means includes a second wire or leaf spring 19 secured to the hub 14 and engaging the face of brush 12 opposite the holder 13 and stressed to maintain the brush 12 in close contact with the brush holder 13. The elements for mounting the brush 12 thus support it for an articulated motion; that is, the brush can rock or move in a vertical plane to accommodate any irregularities in the plane surface of the disc, while the spring 19 restricts its movement to the vertical plane determined by the vertical face of the holder 13.

The contact brush 12 is preferably graphite of an electrographitic grade having a Shore schleroscope hardnessin-the range-20-45 and having a specific reistance in the range of 0.0004 to 0.002 ohm per inch cube. In fabricating the brush 12, the lower edge portion 12b,

which may be flat or tapered, as shown, is coated-with a commercial photographic resist, for example, asphaltumdissolved in turpentine or xylene. The brush element 12 is then provided with alow-resistance metallic coating, for example, a flash cooper coating'and a superposed-- electroplated silver coating of a thickness of the order of 0.001 inch. After coating the. surface of brush 12 other than that coveredwith the resist, it is immersed in a solvent to remove the resist "andthe remaining solvent is then allowed to dry.

The commercial resist obviously keeps the edge portion 12b uncoated and thisluncoatedportion is then.

preferably vacuum impregnated with a liquid capable of hardening into a homogeneous non-conductive sealing and bonding'substancen This vacuum impregnation may be carried out in aconventional manner, for example, the brush 12 may be supported in a' vacuum bell jar and degassed. Thereafter, by external control means, the brush is lowered .so that the uncoated .edge portion 12b is lowered into a container of sealing compound. The sealing compound is not critical, but ithas been-found that almost any liquid material hardening. into .a non-conductive homogeneous mass which ;will.be stable under the environmental. conditions of the finalresistor as a Whole is satisfactory. Specifically, it has been found that satisfactory sealing is provided by the following mixture Parts by weight Water white grade rosin 1 Castor oil 2 N-amyl alcohol 7 With such an impregnated graphite brush it has been found that, in the initial .usageof the resistor, the brush forms on the film strip 11 a lightdeposit of the graphite ing-in period. However, this deposit of the graphite and.

sealing compound mixture during the. breaking-in period does .notmaterially affect the total resistance of the film stripwll. 'The elfect of the impregnation of the edge.

resistor .and its contact brush are not critical but may.

be proportioned in accordance with the desired characteristics, one satisfactory contact brush has beenin the.

form of a slab of graphitehavinga hardness of 35' and a specific resistance .of 0.0004ohmper inch cube, 0.010:

thick, 0.125 high, and0.093 long,- the length being preferably slightly greater than the widthof the metallic film strip. 11. 'The uncoated edge portion 12b has a width of 0.03. as to develop a brush pressure of: 30 to 40 grams.

The constant of the spring 18 was such- It has been found that a metallic film adjustable resistor of the type described has a numberof distinct ad.-

vantages, among which may be mentioned anunusually long life cycle; a high current-carrying and power dissipating rating; an extremely low contact noise and-contcat resistance; and satisfactory operation in dry,- lowpressure, high-altitude atmospheres.-

While there has been-described what is'at present consideredto be the preferredembodiment of the 'inven-' tion, it will be obvious to those skilled in the art thatvarious changes and modifications may be made therein-.4 withoutdepartingfrom-the invent-ion, andit is, there-.-;

fore,. aimed in the appended claimsv to cover all such:-

changes and modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. A metallic film adjustable resistor comprising a form of insulation material having a plate surface; an elongated strip of thin metallic film formed directly on and bonded to said plane surface; a contact brush formed of a flat slab of graphite having a body portion and a tapered edge portion terminating in a substantially line-contact edge, said body portion having a low-resistance metallic coating; and an adjustable holder for moving said brush along said film strip and holding it in a plane substantially normal to said plane surface and to the tangent of the mean path of movement.-

2. A metallic film adjustable resistor comprising a circular disc of insulation material having a plane surface; an elongated arcuate strip of thin metallic film formed directly on and bonded to said plane surface; a contact brush formed of a flat slab of graphite having a body portion and tapered edge portion terminating in a substantially line-contact edge, saidbody portion having a low-resistance metallic coating; and an adjustable radially extending holder for moving said brush along said'film gated strip of thin metallic film formed directly on and bonded tosaidiplane surface; a contact brush formed of a flat slab of graphite having a body portion and a tapered 'edgej-portion terminating in a substantially linecontact edge, said body portion having. a low-resistance metallic 'coating; and an adjustable holder for moving said brush along said film strip and holding it in .a plane substantially normal to. said plane surface andto' the. tangent of the mean path of movement, said holder in.-

cluding spring. means for biasing said brush into vertical.

contact'with said film strip and restricting brush movement to the vertical plane.

4. In a metallic film adjustable resistor, a contactbrush comprising aflat slab, of graphiteihaving .a body portion and a tapered edge portion terminatingin a substantially line-contact. edge, said body portion having a low-resistance metallic .coating and said edge portion being uncoated and impregnated With a hardening .non-conductive sealing substance.

5. In a metallic film adjustable resistor, a contactbrush comprising a flat slab. of. graphite of electrographite grade having a Shore schleroscope hardnessin the range-of 20 to 45. and having. abody portion and a tapered'edge portion terminating in..a substantially line-contact edge, said body portion having aloW-resistance metalliccoating and said; edge portion being uncoated and impregnated with a hardening non-conductive sealing substance...

6. In a metallic film adjustable resistor, a contact brush comprising a. fiat slab of graphite having .a specific resistancein-the range .of 0.0004to 0.002 ohm per inch cube andhaving .a body portion and a tapered edge portion terminating in a substantially line-contact edge, said body portionhaving a low-resistance metallic coating and said edge portion being uncoated andimpregnated-with a hardening.non-conductivesealingsubstance.

7. In a metallic film adjustable resistor, a contactbrush comprising a flat slab of graphite having a specific resistance of 0.0004 ohm-per inch cube anda ,Shore schleroscope hardness of 35 and having a body portion and a tapered edge portion terminating inxa substantially line'- contact edge, said body portion having a low-resistance metallic coating and said edge portion being uncoated and impregnated with a hardeningnon-conductive sealing substance.

8. In a metallic film adjustableresistor, a contact brush comprising a flat slab. of graphite having a body portion and. a tapered edge-portion terminating ina substantially. line-contact edge, saidabody .portion :having a flashed copper;;coating and a superposed silver coating andsaid edge portion being uncoated and impregnated with a hardening non-conductive sealing substance.

9. In a metallic film adjustable resistor, a contact brush comprising a flat slab of graphite having a body portion and a tapered edge portion terminating in a substantially line-contact edge, said body portion having a low-resistance metallic coating and said edge portion being uncoated and vacuum impregnated with a liquid hardening non-conductive sealing substance.

10. In a metallic film adjustable resistor, a contact brush comprising a flat slab of graphite having a body portion and a tapered edge portion terminating in a substantially line-contact edge, said body portion having a low-resistance metallic coating and said edge portion being uncoated and impregnated with a hardening metallic sili- 15 cate solution.

11. In a metallic film adjustable resistor, a contact brush comprising a fiat slab of graphite having a body portion and a tapered edge portion terminating in a substantially line-contact edge, said body portion having a low-resistance metallic coating and said edge portion being uncoated and impregnated with a mixture comprising essentially water White grade rosin 1 part, castor oil 2 parts, and N-amyl alcohol 7 parts.

National Carbon Co., Catalog #30 (1913), Cleveland, Ohio. 

